What Exactly Is a Plankton Bloom?

Plankton are the tiny drifters of the sea—plants (phytoplankton), animals (zooplankton), and bacteria that cannot swim against currents. In UK coastal waters, spring and summer bring longer daylight, warmer surface temperatures, and nutrients stirred up by winter storms. When these conditions align, phytoplankton can multiply explosively, turning clear water green, brown, or even red. A bloom can span from a few metres to hundreds of square miles. Most are harmless, but some species produce toxins or cause oxygen crashes. Understanding them is part of reading the coast well.

The Good, the Bad, and the Colourful

Most blooms are the foundation of life. Diatoms and coccolithophores feed copepods, which feed sand eels, which feed puffins, mackerel, and basking sharks. Coccolithophores—tiny chalk-plated algae—can give the sea a milky turquoise hue, often seen off Cornwall and the Hebrides. But some dinoflagellates are less welcome. Karenia mikimotoi can cause fish kills. Pseudo-nitzschia produces domoic acid, linked to amnesic shellfish poisoning. Alexandrium species produce saxitoxin, which causes paralytic shellfish poisoning. Then there is Noctiluca scintillans, the sea sparkle—harmless to touch but a sign of nutrient-rich water, and famous for its blue glow.

When Blooms Turn Toxic: Shellfish and Public Health

In the UK, harmful algal blooms are taken seriously. Bivalve shellfish—mussels, oysters, scallops, clams, cockles—filter seawater and concentrate toxins in their flesh. Eating contaminated shellfish can cause tingling lips, dizziness, memory loss, or worse. Cooking does not destroy these toxins. That is why commercial shellfish beds are regularly tested and closed when levels exceed safety thresholds. If you gather your own shellfish, never assume it is safe. Check with your local council or the Food Standards Agency for current warnings. The same applies to recreational fishing: keep an eye on local notices at harbour walls and beach car parks.

Discoloured Seas and Glowing Shores

Blooms can paint the sea in strange colours. Red tides—actually rusty brown or orange—come from dense dinoflagellates. Milky seas from coccolithophores. On calm nights, Noctiluca can light up breaking waves with a ghostly blue flash when disturbed. In Wales, Scotland, and the southwest, this bioluminescence draws night-time visitors. You may also see thick foam on the strandline, a residue of decaying plankton. While spectacular, these signs can indicate low oxygen and stressed marine life. If the water smells sulphurous or looks like pea soup, it is best to keep out.

Coastal Effects: Tourism, Fishing, and Wildlife

Blooms ripple through coastal economies. A large Karenia bloom can kill fish and invertebrates, leaving smelly debris on beaches and putting off day-trippers. Shellfish closures hit local fishermen and seafood businesses. Seabirds, seals, and dolphins can suffer if they eat contaminated prey or breathe aerosolised toxins. On the other hand, blooms attract wildlife: basking sharks and minke whales follow the plankton. For visitors, the key is to check before you travel. Some councils issue temporary swimming advisories when irritants or toxins are present. After storms, blooms often disperse, so conditions can change quickly.

How to Enjoy the Coast Responsibly During Bloom Season

Blooms are most common from late spring to early autumn, though they can appear year-round. Here is how to stay safe and still enjoy the seaside:

• Check local warnings before swimming, paddling, or foraging. Beach lifeguards, harbour masters, and council websites often post updates.
• Never eat self-gathered shellfish unless you know the area is monitored and currently open. Cooking does not make toxic shellfish safe.
• Avoid discoloured or foamy water, especially if you have cuts, asthma, or hay fever. Toxins can irritate skin and airways.
• Report unusual sightings—fish kills, glowing water, thick foam—to your local wildlife trust or environmental agency. Citizen records help track blooms.
• For bioluminescence, choose a dark, calm night and watch from the shore. Disturbing the water too much can break the spell—and the organisms.
• Time your visit after a spell of settled weather, when blooms often peak, or after a storm, when they may clear.

Plankton are not the enemy. They are the engine of our seas. With a little local knowledge, you can appreciate their beauty—and give their toxic moments a wide berth.